1. Field of the Invention
The present invention relates to a motor control microcomputer.
2. Description of Related Art
The pulse width modulation (PWM) drive system has conventionally been widely used in the motor control microcomputer which controls the rotation of a motor. In this PWM drive system, current to a motor coil is controlled by turning on and off a switching element included in a motor drive circuit. In the motor control microcomputer using the PWM drive system, each period between timings to turn a switching element on and off is set to be the same, and power supply to a motor coil is controlled by changing the pulse width during which the switching element is turned on, relative to the period. In the motor control microcomputer, an abnormal rotation of a motor may cause a large current to flow to the switching element, thus thermally destructing the switching element. To avoid such problem, there is a known technique for detecting such an abnormal operation and stopping power supplied to the motor in the case of the abnormal operation.
Japanese Patent Application Publication No. 2000-175481 describes such technique. In this technique, first, PWM signals, which control the rotational speed of a brushless motor and a switching element, are monitored. When the rotational speed is lower than a fixed speed and when the frequency of the PWM signal is more than a fixed frequency, it is judged that a rotational abnormality is occurring. Then, the rotation of the motor is stopped.
However, the motor control microcomputer described in Japanese Patent Application Publication No. 2000-175481 is not configured to instantaneously detect a status in which both of a positive-phase signal and a negative-phase signal cause switching elements configuring an inverter to be driven at the same time. Therefore, the motor control microcomputer described in Japanese Patent Application Publication No. 2000-175481 requires a longer delay time to detect abnormality after the occurrence of the abnormality, so that the risks of the thermal destruction of a switching element, and of the damage to a motor in some cases, increase.